1. Field of the Invention
The present invention relates to a vertical heat treatment apparatus and a treatment method using the vertical heat treatment apparatus.
2. Description of the Prior Art
With recent development in integration degree of an LSI, the density of arrangement of, e.g. MOS FET has increased. In the case, a 1M or 4M DRAM, a minimum design width is less than 1 .mu.m, and the thickness of a gate oxide film is less than 200 .ANG.. In the case of a 16M DRAM, the thickness of a gate oxide film is further decreased to about 100 to 150 .ANG.. When a silicon surface is wet-rinsed with HF or HCl, it remains clear for a time just after rinsing. However, the rinsed silicon surface reacts immediately with oxygen or moisture in the air, and a natural oxidation film about 10 .ANG. thick forms on the silicon surface.
In the case of a horizontal furnace, a boat on which semiconductor wafers are mounted is loaded horizontally in a reaction tube heated, e.g. up to 1000.degree. C. In this case, air enters the reaction tube inevitably, because of convection current due to a temperature difference between the inside of the furnace and the outside thereof. Thus, when a wafer is loaded in the horizontal furnace, a natural oxidation film having a thickness of 50 to 100 .ANG. forms inevitably as a result of a reaction between the heated wafer and oxygen in the air. A natural oxidation film is porous and has a low quality. Therefore, when a high-density element is fabricated, in which the thickness of a gate oxide film must be controlled, the horizontal furnace is undesirable because of its structural problems.
On the other hand, less oxygen is taken in a vertical furnace, compared to the horizontal furnace. In the case of the vertical furnace, the thickness of a formed natural oxidation film is small, i.e. 30.degree. to 50.degree. C. Thus, vertical furnaces are now widely used in the fabrication of 1M DRAMs.
In the case of the vertical furnace, however, when 4M or 16M DRAMs that require higher integration are manufactured, it is necessary to prevent the formation of a natural oxidation film due to the air entering at the time of loading/unloading, and the moisture adsorbed on the wafer.
ELECTRONIC MATERIAL, March 1989, pp. 38-39, discloses a structure for strictly controlling the atmosphere in which a wafer is loaded in a vertical furnace.
According to a load-lock method disclosed in ELECTRONIC MATERIAL, a mechanism for vertically moving a boat, and other parts arranged in a lower part of the vertical furnace are arranged within a load-lock chamber. After a vacuum is built in the load-lock chamber, an inert gas such as nitrogen is filled in the chamber. Then, the boat is loaded. Oxidation of the wafer at the time of loading is thus prevented, and formation of a natural oxidation film is effectively suppressed. The load-lock chamber is connected to a sub chamber, whereby the wafer is loaded into and unloaded from the load-lock chamber in a nitrogen atmosphere.
Published Unexamined Japanese Patent Application No. 62-263642 discloses a technique wherein a vacuum sub chamber is connected to a plasma vapor-phase deposition apparatus, and a wafer is prebaked in the vacuum sub chamber to eliminate moisture.
In the above-described prior art, the thickness of the natural oxidation film is reduced to a minimum by loading the wafer in the vertical furnace in the atmosphere free from oxygen. However, with an increase in integration degree of semiconductor devices, the thickness of the oxide film must be controlled more finely, and it is insufficient to suppress the formation of the natural oxidation film at the time of loading the wafer in the vertical furnace. It is necessary to prevent the formation of the natural oxidation film of the thickness of 10 .ANG., which is formed, during the transfer of the rinsed wafer to a heat treatment furnace, owing to the reaction of silicon with oxygen or moisture in the air.
In addition, in treatment apparatuses, other than the oxide film forming apparatus, for forming a polysilicon film or a capacitor film which must have a low contact resistance, it is necessary to remove an unnecessary natural oxidation film and to increase the yield of semiconductor devices.